Ignore machine-check MSRs

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.\"     @(#)unix.4      8.1 (Berkeley) 6/9/93
.\" $FreeBSD$
.\"
.Dd July 15, 2001
.Dt UNIX 4
.Os
.Sh NAME
.Nm unix
.Nd UNIX-domain protocol family
.Sh SYNOPSIS
.In sys/types.h
.In sys/un.h
.Sh DESCRIPTION
The
.Ux Ns -domain
protocol family is a collection of protocols
that provides local (on-machine) interprocess
communication through the normal
.Xr socket 2
mechanisms.
The
.Ux Ns -domain
family supports the
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
socket types and uses
file system pathnames for addressing.
.Sh ADDRESSING
.Ux Ns -domain
addresses are variable-length file system pathnames of
at most 104 characters.
The include file
.In sys/un.h
defines this address:
.Bd -literal -offset indent
struct sockaddr_un {
        u_char  sun_len;
        u_char  sun_family;
        char    sun_path[104];
};
.Ed
.Pp
Binding a name to a
.Ux Ns -domain
socket with
.Xr bind 2
causes a socket file to be created in the file system.
This file is
.Em not
removed when the socket is closed \(em
.Xr unlink 2
must be used to remove the file.
.Pp
The length of
.Ux Ns -domain
address, required by
.Xr bind 2
and
.Xr connect 2 ,
can be calculated by the macro
.Fn SUN_LEN
defined in
.In sys/un.h .
The
.Va sun_path
field must be terminated by a
.Dv NUL
character to be used with
.Fn SUN_LEN ,
but the terminating
.Dv NUL
is
.Em not
part of the address.
.Pp
The
.Ux Ns -domain
protocol family does not support broadcast addressing or any form
of
.Dq wildcard
matching on incoming messages.
All addresses are absolute- or relative-pathnames
of other
.Ux Ns -domain
sockets.
Normal file system access-control mechanisms are also
applied when referencing pathnames; e.g., the destination
of a
.Xr connect 2
or
.Xr sendto 2
must be writable.
.Sh PROTOCOLS
The
.Ux Ns -domain
protocol family is comprised of simple
transport protocols that support the
.Dv SOCK_STREAM
and
.Dv SOCK_DGRAM
abstractions.
.Dv SOCK_STREAM
sockets also support the communication of
.Ux
file descriptors through the use of the
.Va msg_control
field in the
.Fa msg
argument to
.Xr sendmsg 2
and
.Xr recvmsg 2 .
.Pp
Any valid descriptor may be sent in a message.
The file descriptor(s) to be passed are described using a
.Vt "struct cmsghdr"
that is defined in the include file
.In sys/socket.h .
The type of the message is
.Dv SCM_RIGHTS ,
and the data portion of the messages is an array of integers
representing the file descriptors to be passed.
The number of descriptors being passed is defined
by the length field of the message;
the length field is the sum of the size of the header
plus the size of the array of file descriptors.
.Pp
The received descriptor is a
.Em duplicate
of the sender's descriptor, as if it were created with a call to
.Xr dup 2 .
Per-process descriptor flags, set with
.Xr fcntl 2 ,
are
.Em not
passed to a receiver.
Descriptors that are awaiting delivery, or that are
purposely not received, are automatically closed by the system
when the destination socket is closed.
.Pp
The effective credentials (i.e., the user ID and group list) of a
peer on a
.Dv SOCK_STREAM
socket may be obtained using the
.Dv LOCAL_PEERCRED
socket option.
This may be used by a server to obtain and verify the credentials of
its client, and vice versa by the client to verify the credentials
of the server.
These will arrive in the form of a filled in
.Vt "struct xucred"
(defined in
.In sys/ucred.h ) .
The credentials presented to the server (the
.Xr listen 2
caller) are those of the client when it called
.Xr connect 2 ;
the credentials presented to the client (the
.Xr connect 2
caller) are those of the server when it called
.Xr listen 2 .
This mechanism is reliable; there is no way for either party to influence
the credentials presented to its peer except by calling the appropriate
system call (e.g.,
.Xr connect 2
or
.Xr listen 2 )
under different effective credentials.
.Pp
.Tn UNIX
domain sockets support a number of socket options which can be set with
.Xr setsockopt 2
and tested with
.Xr getsockopt 2 :
.Bl -tag -width ".Dv LOCAL_CONNWAIT"
.It Dv LOCAL_CREDS
This option may be enabled on a
.Dv SOCK_DGRAM
or a
.Dv SOCK_STREAM
socket.
This option provides a mechanism for the receiver to
receive the credentials of the process as a
.Xr recvmsg 2
control message.
The
.Va msg_control
field in the
.Vt msghdr
structure points to a buffer that contains a
.Vt cmsghdr
structure followed by a variable length
.Vt sockcred
structure, defined in
.In sys/socket.h
as follows:
.Bd -literal
struct sockcred {
  uid_t sc_uid;         /* real user id */
  uid_t sc_euid;        /* effective user id */
  gid_t sc_gid;         /* real group id */
  gid_t sc_egid;        /* effective group id */
  int   sc_ngroups;     /* number of supplemental groups */
  gid_t sc_groups[1];   /* variable length */
};
.Ed
.Pp
The
.Fn SOCKCREDSIZE
macro computes the size of the
.Vt sockcred
structure for a specified number
of groups.
The
.Vt cmsghdr
fields have the following values:
.Bd -literal
cmsg_len = CMSG_LEN(SOCKCREDSIZE(ngroups))
cmsg_level = SOL_SOCKET
cmsg_type = SCM_CREDS
.Ed
.It Dv LOCAL_CONNWAIT
Used with
.Dv SOCK_STREAM
sockets, this option causes the
.Xr connect 2
function to block until
.Xr accept 2
has been called on the listening socket.
.El
.Sh SEE ALSO
.Xr socket 2 ,
.Xr intro 4
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "An Advanced 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re